This invention relates to a spectroscopic analyzing instrument employing a holographic spherical grating to disperse incident light into its spectrum.
Spectroscopic instruments are used to analyze materials and color by dispersing light into narrow bandwidths and using these narrow bandwidths to measure reflectivity or transmissivity of a sample. Spectroscopic instruments operating in the near infrared range have been particularly useful in analyzing materials not only to determine constituents ofthe material and identify the material, but also to analyze physical properties of the material.
Modern spectroscopic instruments employ a holographic spherical grating to disperse incident light into its spectrum. The grating directs the spectrum toward an exit slit through which a narrow band of the dispersed spectrum passes. By rotating the grating on a selected axis, the center wavelength of the light passing through the exit slit is scanned through the spectrum. In typical instruments, the grating is oscillated at a high speed to enable a rapid analysis of materials.
A holographic spherical grating tends to focus the dispersed spectrum and it is desired to position the grating relative to the exit slit to focus the light at the exit slit to provide a minimum bandwidth of the light passing through the exit slit.
In prior art systems employing a spherical grating, the grating is positioned to focus the light passing through the exit slit at one wavelength of the dispersed spectrum. The arrangement resulted in achieving an optimum focus at the selected wavelength but not an optimum focus throughout the spectrum.